1. Field of the Invention
The present invention relates to a method of making small anhydrous metal terephthalate (metal-TPA) polymeric crystals or fibers from polyester ethylene terephthalate and polyvalent metal compounds. In one embodiment, it involves pyrolysis of passenger vehicle tires. Polyvalent metal-terephthalate polymers are stable, repeating structures built around terephthalate frameworks, with polyvalent metals bonded to the carboxylic acid functions, thereby providing interconnections between the aromatic rings in the polymer chain.
2. Description of the Prior Art
It has been known to convert used passenger vehicle tires into useful carbon black powder, as well as byproducts through a pyrolysis process. See generally U.S. Pat. Nos. 6,835,861 and 6,833,485.
U.S. Pat. No. 4,952,636 discloses the use of polymeric hindered polyvalent metal salts of an aromatic polycarboxylic acid. It discloses zinc terephthalate providing a crosslinking reaction with carboxylated rubber. It also discloses the high-temperature stability of these polymers. See also U.S. Pat. Nos. 4,983,688; 5,164,458; 5,380,593; and 5,475,045.
U.S. Pat. No. 5,164,458 discloses a process for producing calcium terephthalate elongated fibers having an average particle size of about 100 mesh synthesized by an aqueous process.
U.S. Pat. No. 5,380,593 provides 9 examples of multistep aqueous based syntheses, for making calcium as well as zinc and barium terephthalate fibers. It adds the distinction that these are anhydrous fibers or rods and not hydrated plates as produced by simpler aqueous synthesis processes. The distinction is important because the hydrated plates do not function in most polymer systems because the release of water causes problems. Various sizes of the fibers are given, the smallest being 75 microns in length.
U.S. Pat. No. 5,446,112 discloses a multistep synthesis strategy for making metal terephthalates resins. It begins with an aromatic monocarboxylic acid which reacts with a metal oxide. Water is then distilled off followed by a reaction with a dialkyl ester of an aromatic acid. This yields a metal-terephthalate resin and a byproduct alkyl ester. No conditions are given for the final step. The byproduct alkyl ester is distilled away in this strategy. The only information on the form and purity of the resin product is that it contains less than 10 ppm of metal halide.
U.S. Pat. No. 5,254,666 is directed toward a reaction of polyester ethylene terephthalate with metal compounds to recover terephthalic acid, as well as the polyol of the polyester. The upper limit of temperatures employed is 180° C. The temperature cited is well below the melting point of polyester ethylene terephthalate. The examples mention potassium and sodium, which are monovalent and belong to the class known as alkali metals. The patent states that the product powder is dissolved in water. The patent discloses alkaline earth metals, which are polyvalent and would include calcium and magnesium, but the fact that the product dissolved in water, shows that the product could not have been a polyvalent metal polymer as discussed in the present invention.
U.S. Pat. No. 5,545,746 is also directed toward recovery of terephthalic acid, as well as the polyol of the polyester. It extends the temperature range up to 250° C., which is well below the melting point of polyester ethylene terephthalate. The examples only mention potassium and sodium. The patent discloses dissolving the product in water which again shows that the product could not have been a polyvalent metal polymer as discussed in the present invention.
The paper by P. Baker and R. F. Grossman, “Properties and Reactions of Metal Terephthalates,” Journal of Vinyl Technology, Volume 11, No. 2, pp. 59-61, June, 1989 discloses the unique properties of polyvalent metal terephthalates. One unique property is their high thermal stability which results in high decomposition temperatures. Typically polyvalent metal terephthalates do not compose until about 600° C. to 700° C. The paper explains this unusual stability by showing the interlinked structure of the longchain polyvalent metal terephthalate molecules. The paper also makes the sharp distinction that although alkali metal (monovalent) terephthalate salts are water soluble, no polyvalent metal terephthalate exhibits more than trace solubility. Like the high thermal stability, the insolubility in water follows from the interlinked structure.
In spite of the foregoing known methods and compositions, there remains a very real and substantial need for an improved method of making anhydrous, metal terephthalate polymeric crystals to form polyester ethylene terephthalate and polyvalent metal compounds.